REDLANDS -- On cold nights, citrus growers Jerry and Beth Sanders set their alarm to wake them every hour or two so they can check the thermometer, and, if needed, decide what to do to keep their oranges warm.

Even in Southern California, frosty winter weather can threaten fruit and the health of citrus trees, whether planted in big commercial groves in San Timoteo Canyon and Mentone, family groves like the Sanderses' in the Crafton Hills, or in backyards all over the Redlands area.

Families growing oranges, lemons, grapefruit and other subtropical fruit at home, especially those with just one or two trees, probably don't have those tools available.

Still, there are some things home growers can do to protect their trees, according to Chuck Hills, grove manager for Larry Jacinto Farming, which operates 600 acres of citrus and avocado in Redlands and Mentone.

Critical temperatures - say, 29 degrees or colder - are not in the forecast for the next couple of days, Hills said, but it's important to keep track of weather trends and be ready.

Home growers give their trees the best protection before they plant them, he said.

Location is key.

Close to the house is best, or near a wall, because structures absorb heat in the daytime and radiate it at night.

He said home growers can also insulate the trunks of their citrus trees with materials such as cardboard or palm fronds, and then wrap the trunks in plastic.

"You start to worry when you see temperatures below 30 degrees," Hills said. "A lot depends on where you are, and what time it is: If it's 29 degrees at 8 o'clock in the evening, it's going to be a long night. If it's 29 degrees at 5:30 in the morning, you're going to be fine."

Hills got his first exposure to protecting orange trees from frost in the 1970s.

A student at Redlands High School, he was part of a 12-man crew that would fire up smudge pots in the groves when the temperature dropped. It was hard, sooty work, and he missed out on some sleep.

"But it was a great way for a kid to earn extra money for Christmas vacation," he said.

Smudge pots worked well for decades, starting early in the last century, but they are not economically practical anymore, Hills said.

"As time went on and fuel oil got more and more expensive, they quit using smudge pots," he said. "Back in the 1960s, they paid 12 cents a gallon for fuel. Now it's more like three dollars."

Wind machines that draw warmer air from the inversion layer are the norm now, he said. They can raise the temperature by several degrees.

But the Sanderses, who for 12 years have lived on and operated a 5-acre grove in the Crafton Hills area, spend a lot of money when they run their wind machine. Powered by an old Ford flathead gasoline engine, it cost them $1,600 to run it during one particularly cold week several years ago.

"It was well worth it, because it saved our crop - and it was a great crop that year," Jerry Sanders said. "Not exactly practical for the backyard grower, though."

Sanders suggests keeping the soil around citrus trees very moist at night. It might seem counter-intuitive, because water freezes at 32 degrees. But that is exactly the point, he said. Once the water freezes, it will not get any colder - so a blanket of ice is good protection. Even so, on cold nights he sets his alarm to wake him for thermometer checks.

"Last night was very cold," he said on Monday. "I was up several times, but it never dropped below 32 degrees."